Going from an area of low information density to high density
I think there's no scientific way to learn how to do it, at least in my case I'm still learning day after day how to do it, I guess you just need to try how to join, merge vertices, use the knife to create edges, etc... and keep quads as much as possible, then tris if you don't have choice. Actually each case is a particular puzzle.

Duarte has given you links and it explains how to go from 3 edges to 1, etc... I guess it worth a look.

Anyway, to begin I've separated the eye mesh, then I've used the Knife Project tool to print the eye shape on the rocket mesh.